EP2406866A2

EP2406866A2 - Charge management of a battery pack

Info

Publication number: EP2406866A2
Application number: EP10713351A
Authority: EP
Inventors: Sami Rantula; Antti VÄYRYNEN; Risto Komulainen
Original assignee: Finnish Electric Vehicle Technologies Ltd Oy
Current assignee: EUROPEAN BATTERY TECHNOLOGIES OY
Priority date: 2009-03-13
Filing date: 2010-03-10
Publication date: 2012-01-18
Also published as: WO2010103182A3; FI20095262L; RU2011140050A; FI20095262A0; FI123892B; WO2010103182A2

Abstract

Method and apparatus for equalizing the cell-specific charges of a battery pack comprised of cells (C₁ - C_N) connected in series, which battery pack comprises bidirectional power converters (CM) for transferring charges to/from a cell, which power converters comprise primary circuits and secondary circuits that are galvanically isolated from each other, of which secondary circuits each has its own corresponding cell, to which it is connected, and the primary circuits of which power converters are connected to a common DC circuit (power bus, PB), via which charges can be freely transferred from one power converter to another.

Description

CHARGE MANAGEMENT OF A BATTERY PACK

Field of technology

The object of the present invention is a method and an apparatus for implementing charge management in a battery pack, which comprises a number of cells connected in series. More particularly the object of the invention is the implementation of charge management by means of a power bus that connects all the units that handle the charge management specific to a cell or specific to a group of cells, which bus enables the transfer of charge from any cell whatsoever to any cell whatsoever regardless of the size of the battery pack.

Background to the invention and prior art

Battery packs are used, among other things, as an energy store of vehicles (such as e.g. an HEV, hybrid electric vehicle) that utilize electrical power processing. Since the voltage of an individual battery cell e.g. when using modern lithium-based cells, is typically of a magnitude of only approx. 3 volts, a sufficient amount of them must be connected in series in order to produce the source voltage needed for a power supply, which can be e.g. 42V, 120V or 450V.

It is advantageous to select cells that are of as homogeneous quality as possible for the battery pack, to guarantee consistent performance. In practice there are, however, differences in the capacities (ability to absorb a charge) of cells, a consequence of which can be e.g. deep discharge of an individual weaker cell when loading a battery pack or correspondingly overcharging when charging a battery pack. Both cases mean operation outside the characteristic operating range for the cell, which can lead to permanent damage of the cell. The damaging of an individual cell, for its part, weakens the capacity of the whole battery pack and can even result in a need for immediate servicing.

To prevent these types of fault situations, it is advantageous to endeavor to balance the charges of cells, for which a number of prior-art methods are on offer.

For example, patent publication EP0814556 discloses a method wherein shunt resistors are connected in parallel with the cells, for balancing the charges. Overcharging of a cell can be prevented with the method, but it does not offer any help for preventing deep discharge of a cell.

Patent publication US5498950, for its part, discloses a method wherein a separate power source is used, which can be connected by means of relays to charge the desired cell, for achieving charge balancing. Deep discharge can be prevented with the method, but it is not any help against overcharging, and in addition the system is expensive and complex because of the relay provisioning.

A weakness of the aforementioned prior-art charge management methods is either that charge transfer is possible only in one direction and also a system that requires a lot of apparatus (such as relays) and is therefore expensive and complex. Full support of the cells of a battery pack in both a charging situation and a discharging situation is contingent on being able to transfer charges, according to the situation, from stronger cells to weaker ones, or vice versa. This therefore requires the bidirectional transfer of charge, such as will succeed, in itself, e.g. by connecting two DC/DC converters in parallel but in different directions. The solution easily leads to high costs, because converters are needed for each cell. Patent publication US2005/0024015 presents a method in which a type of charge pump is used, with which pump a charge pulse can be transferred from one cell to the next cell. According to the method, full charge management is in principle possible, but in practice only if a weak and a strong cell are situated close to each other. Patent publications US4600984 and US7046525 have made known to the art a bidirectional flyback converter, which offers one advantageous opportunity for transferring charges in both directions. In patent publication US2008/0272735 this type of method is, in fact, applied to a cell group, in which charges can be transferred inside a group. When a battery pack comprises a number of cell groups, neither does this method offer an opportunity to equalize charges when a weak and a strong cell are situated in different groups.

Summary of the invention

The aim of this invention is to achieve a method and an apparatus for charge management, with which the aforementioned drawbacks are avoided and the transfer of charges from any cell whatsoever to any other cell whatsoever is enabled regardless of the number of cells that are connected in series. This aim is achieved with the method and the apparatus according to the invention, which are characterized by what is disclosed in the characterization parts of the independent claims. The other preferred embodiments of the invention are the objects of the non-independent claims.

According to the invention, converters that enable bidirectional charge transfer are used either for specific cells or for specific cell groups, in which converters transformers are used, the primary winding of which transformers is galvanically isolated from the secondary winding (secondary windings). The invention does not place limitations on the operating principle of a converter, it can be any bidirectional type whatsoever that enables the transfer of charge/power, preferably e.g. a bidirectional flyback. The circuits connected to the primary windings of all the converters are, according to the invention, connected to the same power bus, along which the transfer of charge/power from any cell/cell group whatsoever to any other cell/cell group whatsoever is possible.

The voltage of the power bus can be e.g. approx. 12VDC or 24VDC. According to one preferred embodiment of the invention a power bus is connected directly, or via a resistor/diode, over e.g. four cells connected in series (12VDC bus) or over eight cells connected in series (24VDC bus). The bus can also be completely separate, without any galvanic connection to the battery. From the viewpoint of the insulation between the primary and the secondary, it can be advantageous to bind the potential of the bus to the midway point of the battery pack, in which case the necessary insulation level is only one-half compared to if the bus were bound to the other the other end of the battery pack, e.g. to the negative pole.

According to an essential characteristic feature of the invention, the management system of the battery pack ensures that that sum of the charges transferred to the power bus and taken from it remains on average zero, i.e. the sliding time integral of the charge pulses always remains close to zero. As a result of this, the voltage of the bus remains essentially constant, and if the bus is e.g. galvanically connected directly over some cell group, the state of charge of the cell group in question will not start to drift in any direction whatsoever because of the connection to the power bus.

The method and the apparatus of the invention enable full charge management of a battery pack regardless of the number of cells connected in series, which is a prerequisite for exploiting the full performance capability of a battery pack. The method is easy to implement and the apparatus required by it is simple and inexpensive in terms of costs.

Short description of the drawings

In the following, the invention will be described in more detail by the aid some embodiments with reference to the attached drawings, wherein

Fig. 1 presents a battery pack comprised of cells connected in series,

Fig. 2 contains an example of a characteristic attribute curve of a battery cell,

Fig. 3 contains an example of a prior-art solution for equalizing the charges of battery cells, Fig. 4 contains an embodiment of a solution according to the invention for equalizing the charges of battery cells,

Fig. 5 presents a second embodiment of a solution according to the invention for equalizing the charges of battery cells,

Fig. 6 presents a bidirectional flyback converter, and Fig. 7 describes an example of the current pulses of a power bus and the time integral of them.

Detailed description of the invention

Fig. 1 presents a battery pack comprised of cells Ci - CN connected in series, the poles of which battery pack that are connected to an external charging/load circuit are B+ (positive pole) and B- (negative pole). The number N of cells can be hundreds, depending on what the desired voltage U₅ of the battery pack is. The optimal loading of a battery requires that the states of charge (SoC, State of Charge) of the cells are near to each other, which is generally seen also in that the voltages U_Ci - U_CN of the cells are of roughly the same magnitude. An external charging/load current i_B travels through all the cells, thus producing a change of the same magnitude in the charge (charge is a time integral of current) of all the cells connected in series.

Fig. 2 presents the characteristic behavior of cell voltage as a function of the charge remaining in a cell. The voltage has a certain lower limit UMIN, a voltage below which means a so-called deep discharge, in which state operation can cause permanent damage. The cell voltage also has a certain upper limit UMAX, charging to a higher voltage than which can also cause permanent damage. Normally, therefore, the aim is to operate between these voltage limits. The figure presents two characteristic curves of cells that differ from each other in their charge capacity, in which the full capacity of cell 1 is Q_Ci and correspondingly of cell 2 is slightly higher Q_C2- Based on the figure, it can easily be seen that if both cells started to be charged from the deep discharge limit (state of charge 0) with the same current, charging would have to be stopped when cell 1 reached its full charge. In order for cell 2 to be fully charged also, to exploit the full capacity of the whole battery pack, some kind of balancing system must be used between the cells, which will transfer the charge from cell 1 to cell 2. Correspondingly, conversely when loading both fully-charged cells with the same current, the charge must be transferred from cell 2 to cell 1 when approaching the deep discharge limit in order to preserve the charge balance.

Fig. 3 presents a known principle, with which overvoltage of the cells when charging a battery pack can be prevented. The apparatus comprises a relay (S₁ - S_N) and a resistor R_BAL serial circuit connected in parallel with each cell. If there is a danger of overcharging some cell, the resistor in parallel with it is connected, in which case a part of the charge of the cell in question is discharged into the resistor.

This method does not, however, help to prevent deep discharge of a cell that is weaker than the others, but instead that must be handled e.g. by stopping the loading of the battery pack within the time set by the weak cell.

Fig. 4 is an embodiment of a system according to this invention for preserving the charge balance of accumulator cells. For example, a balancing unit CM enabling bidirectional charge transfer is connected to each cell. A power converter implements the charge transfer, which power converter comprises primary circuits and secondary circuits that are galvanically isolated from each other, which primary circuit is disposed in the potential Pp₈ of the external power bus and which secondary circuit is disposed in the potential Pc of a cell. The power bus PB thus connects all the balancing units CM. The voltage of the power bus preferably corresponds to the voltage of a few cells connected in series, e.g. according to the figure to the voltage (approx. 12VDC) of four cells. The power bus can be floating, fully detached from the battery pack, or it can be connected to some cell group, e.g. by means of the connection leads 1 and 2 presented with dashed lines in the figure. The voltage of the bus can also be controlled to be higher than the voltage of the cell group connected to it, in which case the charge pulses of the bus do not circulate via the cell group. In this case e.g. a diode V_PB according to the figure can be used, which diode prevents discharge of the higher voltage of the power bus into the lower voltage of the cell group.

According to the invention, the control unit CU that manages the operation of the battery pack controls the operation of the balancing units CM via the control bus CB so that the sum of the charges transferred from the cells to the power bus and from the power bus to the cells is on average 0. By doing so, when the power bus is connected to some cell group C_Gi, the charge balance of the cell group in question is not disturbed on account of the connection to the bus. The charge balance of the power bus can be supervised by measuring the current of one or other of the connection leads (1 , 2), e.g. by means of the shunt resistor R_PB of Fig. 4, which shunt resistor is presented with dashed lines as an alternative circuit. If the power bus is detached from the battery pack or of higher voltage than the group connected to it via the diode Vp_B, which is presented with dashed lines as an alternative circuit, corresponding supervision of charge balance can be effected by monitoring the voltage of the power bus; in a balanced situation the voltage remains within the set limits.

Fig. 5 presents another embodiment of a system according to the invention. Here, the balancing units CGM are specific to a cell group; in the case of the figure, the battery pack is divided into groups of cells CG_I - CG_N, of which each group has its own shared unit. The operation is otherwise the same as in the case of Fig. 4, but now the balancing unit comprises as many secondary circuits as there are cells connected to the unit, i.e. in this case 4 secondary circuits. An advantage of this type of system is lower costs than the costs of the cell-specific system of Fig. 4. The number of cells belonging to a group can, of course, also be other than 4. The maximum amount is limited mainly by problems connected to the implementation, such as e.g. the number of secondary windings that fit onto the same frame in the transformer of a power converter.

The control unit CU, the control bus CB, the current measuring shunt RPB and the voltage separation diode V_PB can also in the embodiment of Fig. 5 have the same importance as in the embodiment of Fig. 4; they are only omitted for the sake of the clarity of the drawing. The control unit CU can be a separate unit or it can preferably be integrated into some balancing unit CGM that is more extensive than the others.

Fig. 6 presents in simplified form one preferred opportunity of implementing the charge transfer converter relating to the invention. Shown here is a so-called bidirectional flyback converter, which comprises a primary circuit (primary winding Np, primary switch V_P) and one (relating to the embodiment of Fig. 4) or more (relating to the embodiment of Fig. 5) secondary circuits (secondary windings Nsi - N_SN, secondary switches V_Si - V_SN)- The switches are preferably MOSFET transistors, the internal construction of which as is known in the art also comprises a diode. A capacitor C_PB can be connected to smooth the voltage of the power bus interface of the primary circuit, which capacitor has an advantageous importance particularly when the power bus is isolated from the battery pack or its voltage is higher than the voltage of the cell group connected via a diode to it.

When it is desired to transfer a charge pulse from the power bus to a cell with the converter according to the figure, the primary switch V_P is first controlled to be conductive. The current of the primary winding of the flyback transformer in this case increases linearly until the switch is controlled to be non-conductive. The energy charged into the magnetic circuit of the transformer T₁ discharges after this into that secondary circuit that has the lowest voltage. If no secondary switch is controlled to be conductive, energy discharges via the internal diodes of the secondary FETs into the cell that has the lowest voltage. By using power FETs, the channel resistance if which is so small that the voltage of their conductive state is clearly lower than the drop-out voltage (approx. 0.7 V) of the internal diode of the FET, e.g. 0.2V, it is possible with the control of the secondary switches to select into which cell the energy of the magnetic circuit of the transformer is discharged, regardless of the small voltage differences of the cell voltages.

Correspondingly, when it is desired to transfer a charge pulse from a cell to the power bus with the converter, the corresponding secondary switch Vs corresponding to the desired cell is first controlled to be conductive. The current of the secondary winding in this case increases linearly until the switch is controlled to be non-conductive. The energy of the magnetic circuit of the transformer T₁ discharges after this into the primary circuit (= power bus) via the internal diode of the primary switch. Fig. 7 presents the characteristic behavior of the current i_PB of a power bus and of its time integral jip_B when using a flyback converter according to Fig. 6 and a zero-sum method of charge transfer according to the invention. The waveform of the current i_PB is triangular in nature, and since the management system of the battery pack ensures that the surface areas of the positive and negative current pulses inside any reasonably-dimensioned time window whatsoever, e.g. 1 ms, remain of equal magnitude, the time integral ji_PB of the current also stays reasonably close to zero (= sliding average value is zero). This is a prerequisite for the charge balance of the cell group connected to the bus not starting to drift in any direction whatsoever, and for the voltage of the bus, which is detached from the battery pack or connected to the battery cell group, but of higher voltage, to stay inside the desired limits.

It is obvious to the person skilled in the art that the different embodiments of the invention are not limited solely to the examples described above, but that they may be varied within the scope of the claims presented below.

Claims

1. Method for equalizing the cell-specific charges of a battery pack comprised of cells (Ci - C_N) connected in series, in which battery pack bidirectional power converters are used for transferring charges to/from a cell, characterized in that the power converters comprise primary circuits and secondary circuits that are galvanically isolated from each other, of which secondary circuits each has its own corresponding cell, to which it is connected, and the primary circuits of which power converters are connected to a common DC circuit (power bus, PB), via which charges can be freely transferred from one power converter to another.

2. Method according to claim 1 , c h a ra c te ri ze d in that the transfer of charges is controlled so that the sliding time average of the sum of the charges transferred to the power bus from the battery cells (Ci - C_N) and from the power bus to the battery cells is essentially zero.

3. Method according to claim 1 or 2, c h a ra cte rized in that the power bus is connected to a positive and to a negative pole of some cell group (CQI - CQN), which comprises at least two cells, and the current transferred from the cell group to the power bus is measured and the measurement result is used for controlling the charge balance of the power bus.

4. Method according to claim 1 or 2, c h a racte ri zed in that the power bus is galvanically isolated from the battery pack, and the voltage of the power bus is measured and the measurement result is used for controlling the charge balance of the power bus preferably so that the voltage of the power bus remains essentially constant.

5. Method according to claim 1 or 2 c h aracterized in that the power bus is connected via a diode (V_PB) to a positive and to a negative pole of some cell group, and the voltage of the power bus is measured and the measurement result is used for controlling the charge balance of the power bus preferably so that the voltage of the power bus remains essentially constant and higher than the voltage of the cell group in question.

6. Apparatus for equalizing the cell-specific charges of a battery pack comprised of cells (Ci - CN) connected in series, which battery pack comprises bidirectional power converters for transferring charges to/from a cell, ch aracterized in that the power converters comprise primary circuits and secondary circuits that are galvanically isolated from each other, of which secondary circuits each has its own corresponding cell, to which it is connected, and the primary circuits of which power converters are connected to a common DC circuit (power bus, PB), via which the control unit (CU) managing the battery pack is fitted to control the transfer of charges from one power converter to another.

7. Apparatus according to claim 6, c h a r a c t e r i z e d in that the control unit (CU) managing the battery pack is fitted to control the transfer of charges so that the sliding time average of the sum of the charges transferred from the battery cells to the power bus and from the power bus to the battery cells is essentially zero.

8. Apparatus according to claim 6 or 7, c h a racte rized in that the power bus is connected to a positive and to a negative pole of some cell group (C_Gi - C_GN), which comprises at least two cells, and the control unit managing the battery pack is fitted to measure the current transferred from the cell group to the power bus and to use the measurement result for controlling the charge balance of the power bus.

9. Apparatus according to claim 6 or 7, c h a racte rize d in that the power bus is galvanically isolated from the battery pack, and the control unit managing the battery pack is fitted to measure the voltage of the power bus and to control the charge balance of the power bus on the basis of the measurement result so that the voltage of the power bus remains essentially constant.

10. Apparatus according to claim 6 or 7, characterized in that the power bus is connected via a diode (V_PB) to a positive and to a negative pole of some cell group, and the control unit managing the battery pack is fitted to measure the voltage of the power bus and to control the charge balance of the power bus on the basis of the measurement result so that the voltage of the power bus remains essentially constant and higher than the voltage of the cell group in question.

11. Apparatus according to any of claims 6- 10 above, c h a racterized in that the power converter is a bidirectional flyback converter, which comprises a primary circuit that comprises a primary winding (N_P) and at least one primary switch (Vp), and one or more secondary circuits that comprise one or more secondary windings (Nsi - NSN) and at least one secondary switch (V_Si - V_SN).